Need help figuring out why a PC shuts down

[Blackhawk3339]

I have a PC that keeps shutting itself off. It doesn't reboot, it shuts down as if someone had unplugged the cord. Once it happens, pushing the power button doesn't do anything for several minutes. After the wait, I can try to boot again.

Sometimes it happens within seconds of boot, sometimes when Windows hits the desktop. Sometimes it sits there for an hour, then shuts down as soon as I start using it. Other times it works all day long.

It has plenty of ventilation. All of the fans are working, and work up and through the crash. I've sat there and watched the CPU fan keep spinning while it shuts down.

It isn't lack of power. The PSU is a 750 watt, which is overkill for the hardware.

It can't be heat, as the system is clean and it frequently happens on the first boot of the day before it even starts loading Windows.

I've swapped out the RAM, running different sticks from different manufacturers in different slots.

I've tried swapping out the PSU for a 400 watt I had sitting around.

I've tried removing extraneous cards (sound, network.)

I've tried removing all drives except for the system drive.

I've swapped out IDE/SATA cables.

I'm at a loss for what else to try.

 

[C1]

What you are reporting is a phenomena experienced with certain HP noebooks. When that happens, it is a problem with the MB.

In your case though, I would try this first. Pull the MB out of the case and run it with just a minimum of components/peripherals needed. There's an outside chance that there is an installation issue between the MB and case mounting. If the system runs satisfactory, then begin adding components back on line. The final step is to careful reinstall the system in the case, but follow your MB manual for the procedure/technique.

 

[sm625]

Remove your front panel switch wiring. It is possible that the power switch is simply shorting itself intermittently thus turning the pc off "randomly". Use a pencil or screwdriver to short the two power switch pins on the motherboard, or simply just unplug this header once you turn the pc on.

If that doesnt work then remove ALL front panel wiring. Yes, I have had a usb port short out which will shut you down. Take it all out if you have to.

 

[Blackhawk3339]

Making less and less sense to me. I yanked everything. I have the motherboard sitting on an insulator on a chair with nothing attached to it, anywhere, except the power supply. As soon as I plug in the PS, the mobo fans all spin up as if it is trying to boot, with nothing on the power switch leads and no attempt to short the pins.

 

[ch33zw1z]

Do you have the VGA, RAM, and CPU installed?

If yes, reseat VGA and RAM. If no, install VGA, RAM, and CPU

edit: What are the hardware specs? Is CPU HSF securely installed?

 

[Blackhawk3339]

It's a Microstar K8N Neo4-F
I't an Athlon 64 CPU, but I can't tell you exactly which model, as I can't start the system.

Everything has been seated and re-seated a dozen or more times, as well as switched out with alternate components, as I mentioned earlier.

 

[ch33zw1z]

But now you're saying that the system is giving "No POST, No Video" while it's out of the case and only have critical components installed. If that's the case, then the symptom has changed from shutting down on use, and now is at "No POST, No Video"...correct?

And you've already reseated the CPU and HSF?

also, is there a speaker on board to hear POST errors? Or do you have a speaker to plug into the header to hear POST errors?

 

[Blackhawk3339]

Actually, what I said was that the mobo was powering all the way up without having had the 'power up' switched tripped.

In any case, it is a mood point. More testing and inspection has confirmed that the mobo is, indeed, dead.

 

[Matt1970]

The most likely culprit of instant power failures like that is the power supply.